The objectives of this research project are twofold: (a) To discover the molecular basis for genetic control in the lactose operon by x-ray crystallographic and physico-chemical studies of the repressor-operator complex; and (b) To understand the folding mechanism and evolutionary development of the cytochromes c and of the respiratory and photosynthetic electron transport chains of which they are a part. Using pure lac repressor prepared by new methods developed in this laboratory, and lac operator from plasmid-bearing strains of E. coli developed in San Francisco from synthetic operator prepared here, we plan to study the binding of repressor to operator and to prepare crystalline complexes for x-ray structure analysis. Optimized atomic coordinates for tuna oxidized and reduced cytochrome c have been obtained by computerized fitting to the electron density maps at 2.0 A resolution, and we plan to continue with difference map and phase refinement using 1.5 A data. Continuing our study of bacterial cytochromes c that has included c550 from Paracoccus and c551 from Pseudomonas, we presently are crystallizing c3 from Desulfovibrio vulgaris for a structure analysis of a four-heme, low-potential cytochrome. BIBLIOGRAPHIC REFERENCES: R.E. Dickerson, R. Timkovich and R.J. Almassy (1976), J. Mol. Biol. 100, 473-491. "The Cytochrome Fold and the Evolution of Bacterial Energy Metabolism." Timkovich, R.E. Dickerson and E. Margoliash (1976), J. Biol. Chem. 251, 2197-2206. "The Amino Acid Sequence of Paracoccus denitrificans Cytochrome c 550."